[net-next] virtio: change comment in transmit

The original comment was not really informative or funny
as well as sexist. Replace it with a better explanation of
why the driver does stop and what the impacts are.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

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From: Stephen Hemminger <stephen@networkplumber.org>
Date: Tue, 24 Mar 2015 16:22:07 -0700

> The original comment was not really informative or funny
> as well as sexist. Replace it with a better explanation of
> why the driver does stop and what the impacts are.
> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Applied, but _only_ because it adds clarity.

I absolutely do not believe in this for the sake of making people
easily made upset feel better, that's censorship and it's crap.
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Stephen Hemminger <stephen@networkplumber.org> writes:
> The original comment was not really informative or funny
> as well as sexist. Replace it with a better explanation of
> why the driver does stop and what the impacts are.
>
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Fair call.  Comment was certainly snarky, probably sexist.  I think it
expressed my feelings perfectly, however.

I note that there's still no comment saying "don't do this" in
netdevice.h; I gather returning NETDEV_TX_BUSY is still considered a Bad
Thing?

(Does it really BUG_ON?)

Thanks,
Rusty.

> --- a/drivers/net/virtio_net.c	2015-03-24 15:20:25.174671000 -0700
> +++ b/drivers/net/virtio_net.c	2015-03-24 16:17:28.478525333 -0700
> @@ -939,8 +939,12 @@ static netdev_tx_t start_xmit(struct sk_
>  	skb_orphan(skb);
>  	nf_reset(skb);
>  
> -	/* Apparently nice girls don't return TX_BUSY; stop the queue
> -	 * before it gets out of hand.  Naturally, this wastes entries. */
> +	/* It is better to stop queue if running out of space
> +	 * instead of forcing queuing layer to requeue the skb
> +	 * by returning TX_BUSY (and cause a BUG message).
> +	 * Since most packets only take 1 or 2 ring slots
> +	 * this means 16 slots are typically wasted.
> +	 */
>  	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
>  		netif_stop_subqueue(dev, qnum);
>  		if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
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From: Rusty Russell <rusty@rustcorp.com.au>
Date: Wed, 25 Mar 2015 14:29:38 +1030

> I note that there's still no comment saying "don't do this" in
> netdevice.h; I gather returning NETDEV_TX_BUSY is still considered a Bad
> Thing?

You're not supposed to do it still, that's right.

If the driver returns NETDEV_TX_BUSY it means that it is not
maintaining the TX queues stop/start state properly.

It was kept around for drivers that we didn't want to convert
or fix up at the time (this is circa two decades ago).

I realize that there are situations for some devices where this is
difficult to achieve, but the stack really does a non-trivial amount
of useless work if you don't play along nicely.

If you grep drivers/net for it, the majority of the returns are in
assertion failure paths in the TX handler of those drivers.
NETDEV_TX_BUSY is not being returned in a normal path of operation.

> (Does it really BUG_ON?)

No, it doesn't.
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